This invention relates to solid polymer electrolytes suitable for use with primary batteries, secondary batteries, electrochromic displays, electrochemical sensors, ion-tophoretic devices, capacitors and other electrochemical devices.
Conventional solid polymer electrolytes are generally in the form of compositions that contain lithium perchlorate and other salts in thermoplastic polyethers such as high-molecular weight polyethylene oxide, three-dimensional cross-linked compounds of polyethers such as polyethylene oxide, and graft copolymers having linear polyethylene oxide side chains in backbone chains such as polyphosphazine chains, polysiloxane chains, polyethylene chains and polypropylene chains.
Thermoplastic polyethers such as high-molecular weight polyethylene oxide and the above-mentioned graft copolymers both have low mechanical strength and their ion conductivity is also low at temperatures lower than room temperature. The mechanical strength and ionic conductivity at low temperatures of polyethers might be enhanced by crosslinking them into a three-dimensional network. Polyethers such as polyethylene oxide are capable of solubilizing large amounts of salts such as lithium perchlorate but they have a great tendency to form complexes with a lithium ion, causing crystallization through strong binding. In an attempt at improving the ionic conductivity of polyethers such as polyethylene oxide at low temperatures, it has recently been proposed that they be dissolved uniformly in solvents such as propylene carbonate together with salts to form three-dimensional crosslinked compounds. However, the incorporation of solvents in large amounts has lead to a lower mechanical strength.